In exposure apparatuses that are used in the manufacturing of, for example, semiconductor devices and liquid crystal display devices, many linear motors, which are capable of noncontactual drive, are used as drive apparatuses of, for example, reticle stages on which masks (reticles and the like) are mounted and wafer stages on which photosensitive substrates (wafers, glass plates, and the like) are mounted.
This type of linear motor comprises a structural body that has a group of coil bodies and rows of permanent magnets that sandwich the coil bodies from above and below; in addition, the permanent magnets are fixed by, for example, bonding them to a magnetic body, i.e., a yoke, so as to hold them and form magnetic paths through which their magnetic flux passes. Furthermore, the linear motor is configured so that it obtains drive power (thrust) produced from the Lorentz force generated by energizing the coil bodies, which are positioned in the magnetic field produced by the permanent magnets.
The lines of magnetic force that are generated by the coil bodies act on a casing, which houses the coils, as well as on the permanent magnets and the yoke that supports such; however, if a fluctuating magnetic field acts on the casing, which is a conductor, the permanent magnets, or the yoke, then eddy currents will arise in the vicinity of the surface of the conductor, and those eddy currents counteract the magnetic field fluctuations. Therefore, Patent Document 1 discloses a technology wherein the coil body casing is formed from a highly conductive plate material and an opening is provided therein in order to adjust the value of the resistance to the eddy currents.
Patent Document 1 Japanese Unexamined Patent Application, Publication No. H11-196561